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ScienceWeek
ATMOSPHERIC SCIENCE: ON WATER IN EARTH'S ATMOSPHERE
The following points are made by Karen H. Rosenlof (Science 2003 302:1691):
1) The air in Earth's stratosphere (the atmospheric layer from ~11 to ~50 km) is extremely dry. The causes of this aridity have been studied for over half a century. Brewer provided a general explanation in 1949 (1), but new measurements reported Webster and Heymsfield (2) have shed new light on the underlying mechanisms.
2) In the 1940s, aircraft measurements showed unexpectedly low amounts of stratospheric water vapor over England. The water vapor content was below 5 ppmv (parts per million by volume), much less than if the air were saturated at the temperature of the local mid-latitude tropopause (the boundary between the stratosphere and the underlying troposphere). Brewer concluded that all air entering the stratosphere must be freeze-dried near the tropical tropopause, where temperatures are regularly below 195 K (1).
3) This overall view of stratospheric circulation has been confirmed by observations of other chemical species and through dynamical studies. However, there exists a quantitative mismatch. The stratosphere contains less water than it would if the air entered at the average minimum tropical temperature. To explain this observation, Newell and Gould-Stewart(3) hypothesized in 1982 that air only enters the stratosphere at the coldest locations and times of the year. They argued that a "stratospheric fountain" exists over the Indonesian maritime continent during Northern Hemispheric winter.
4) However, there are conceptual problems with the fountain mechanism. Satellite data show that air enters the stratosphere throughout the year(4). The stratospheric fountain idea is also contradicted by observational evidence that the average motion in the lower stratosphere is downward over the Indonesian subcontinent (5). Furthermore, long-term increases in mid-latitude lower stratospheric water vapor have been observed while the tropical tropopause and lower stratosphere have cooled. If the fountain mechanism were the only important process acting, one would expect a decrease in water vapor coupled with a temperature decrease. Hence, other processes must also be important.
5) The relation between temperatures near the tropical tropopause and stratospheric entry conditions for water is thus not as simple as postulated by Brewer. Researchers are therefore looking for a dehydration mechanism that explains both the mean values and temporal changes observed in stratospheric water vapor. Two mechanisms are currently considered to be the leading candidates for tropical tropopause layer dehydration. One involves convective processes that loft ice into the upper troposphere/lower stratosphere (UTLS). The second assumes that air parcels gradually ascend, eventually passing though the region of coldest temperatures in the tropical tropopause layer. Two-step processes are also possible, with convection lofting parcels and ice to the upper troposphere, followed by gradual dehydration as the parcels ascend into the stratosphere. If only one mechanism is active, it should leave a distinct isotopic signature in the water vapor that enters the stratosphere.
References (abridged):
1. A. W. Brewer, Q. J. R. Meteorol. Soc. 75, 351 (1949).
2. C. R. Webster, A. J. Heymsfield, Science 302, 1742 (2003).
3. R. W. Newell, S. Gould-Stewart, J. Atmos. Sci. 38, 2789 (1982).
4. P. W. Mote et al., Geophys. Res. Lett. 22, 1093 (1995) [ADS].
5. S. C. Sherwood, Geophys. Res. Lett. 27, 677 (2000).
Science http://www.sciencemag.org
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ATMOSPHERIC SCIENCE: ON THE STRATOSPHERE AND WEATHER
The following points are made by M.P. Baldwin et al (Science 2003 301:317):
1) Is the stratosphere, the atmospheric layer between approximately 10 and 50 km, important for predicting changes in weather and climate? The traditional view is that the stratosphere is a passive recipient of energy and waves from weather systems in the underlying troposphere, but recent evidence suggests otherwise, indicating that the stratosphere responds to forcing from below, initiating feedback processes that in turn alter weather patterns in the troposphere.
2) The lowest layer of the atmosphere, the troposphere, is highly dynamic and rich in water vapor, clouds, and weather. The stratosphere above it is less dense and less turbulent. Variability in the stratosphere is dominated by hemispheric-scale changes in airflow on time scales of a week to several months. Occasionally, however, stratospheric air flow changes dramatically within just a day or two, with large-scale jumps in temperature of 20 K or more.
3) The troposphere influences the stratosphere mainly through atmospheric waves that propagate upward. Recent evidence shows that the stratosphere organizes this chaotic wave forcing from below to create long-lived changes in the stratospheric circulation. These stratospheric changes can feed back to affect weather and climate in the troposphere.
4) Throughout northern-hemispheric winter, air flowing over mountain ranges and continental land masses induces large planetary-scale waves in wind patterns that propagate upward, refract, and reflect in the stratosphere. The waves break in the stratosphere and above, analogous to ocean waves breaking on a beach. They thereby create fluctuations in the strength of the polar vortex formed by high-latitude winds in winter. These fluctuations tend to move down to the lowermost stratosphere, where they can last 1 to 2 months.
5) The Southern Hemisphere has fewer mountain ranges and less land surface. Hence, the planetary-scale waves there are smaller in amplitude, and wave forcing of the stratosphere is less influential than in the Northern Hemisphere. Consequently, the southern-hemispheric polar vortex is relatively undisturbed during winter and early spring, and large stratospheric vortex variations occur mainly during late spring.
Science http://www.sciencemag.org
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ON THE DISCOVERY OF THE STRATOSPHERE
The atmosphere of Earth is divisible into several layers, each layer having a characteristic temperature range, pressure range, and composition. The layers, from the surface of Earth, are (with thicknesses varying at different latitudes): troposphere (0 to approximately 10 kilometers), stratosphere (from approximately 10 to 50 kilometers), mesosphere (approximately 50 to 80 kilometers), thermosphere (approximately 80 to 500 kilometers), and exosphere (above approximately 500 kilometers. Other layers, essentially meta-layers, are also recognized: a) the "chemosphere" is the region between approximately 32 and 92 kilometers where many important chemical reactions occur; b) the "ionosphere", above approximately 80 kilometers, is a shell of high electron concentration resulting from very short wavelength sunlight stripping electrons from atoms and molecules (mainly oxygen and nitrogen) to create an ionized layer; c) the magnetosphere is the constantly changing magnetic field generated by the Earth's dynamo, this magnetic field influencing the behavior of electrically charged particles, and the field extending approximately 10 Earth radii (64,000) kilometers into space on the sunward side.
The boundary between troposphere and stratosphere is called the "tropopause"; that between stratosphere and mesosphere is called the "stratopause"; and that between mesosphere and thermosphere is called the "mesopause", in each case the root "pause" used because of an inflection in the temperature-altitude curve.
The temperature of the atmosphere undergoes marked but systematic variation with altitude. In the troposphere, the layer closest to the surface, the temperature decreases by approximately 6.5 degrees centigrade per kilometer of altitude, until at the tropopause (10 to 11 kilometers) the temperature stabilizes at approximately -53 degrees centigrade. The temperature remains stable in the stratosphere, and even increases with altitude to approximately 0 degrees centigrade at the stratopause. Then in the mesosphere there occurs again a decline in temperature with altitude, now down to -100 degrees centigrade, and then after the mesopause and into the upper atmosphere (thermosphere and exosphere), the temperature rises markedly in these regions of extremely low air density, so that at 200 kilometers the temperature range is 300 to 900 degrees centigrade, depending on solar radiance.
The first hint that Earth's atmosphere is a series of concentric shells was provided by the meteorologist Leon Teisserenc de Bort (1855-1913) [the surname is Teisserenc de Bort]. From 1892 to 1896, Teisserenc de Bort served as chief meteorologist at the Central Meteorological Bureau in Paris, but in 1896 he resigned and carried out his meteorological balloon investigations himself at his estate near Versailles. He conducted experiments with high-flying instrumented balloons, and he was one of the pioneers in the use of such devices. He discovered that above approximately 11 kilometers the temperature, which drops steadily from sea-level to that altitude, remained constant up to the highest points he could reach. Surprised by this result, he accumulated data from 236 balloon ascents before he suggested, in 1902, that the atmosphere was divided into 2 layers. During the next few years, he termed the lower layer, the layer involving air movements, the "troposphere" ("sphere of change"), and the layer above that, a layer he mistakenly thought consisted of internal further layers, the "stratosphere" ("sphere of layers"). Thus, to Teisserenc de Bort we owe both the discovery and the name of the stratosphere.
The following points are made by Mott T. Greene (Nature 2000 407:947):
1) The author suggests that ripples from Teisserenc de Bort's discovery of the stratosphere spread far beyond meteorology. Between 1902 and 1904, the oceanographer Vagn Ekman (1874-1954) discovered similar layering of the ocean, and in 1909, the meteorologist Andrija Mohorovicic (1857-1936) used seismology to establish the existence of a similar discontinuity in the solid Earth, the discontinuity now known as the "Moho".
2) The author (Greene) also suggests that the discovery that the Earth-ocean-atmosphere system is composed of concentric shells of different density, from the core of the Earth to the top of the atmosphere, is the founding insight of modern geophysics, and that the discovery also profoundly influenced the thinking of the young meteorologist Alfred Wegener (1880-1930), leading Wegener in 1912 to propose the theory of continental drift, with the continents representing the remains of a formerly continuous Earth shell above the ocean floors. Greene concludes: "Just as air masses and ocean water masses moved under the influence of the Earth's rotation, sliding along surfaces of discontinuity, so, he [Wegener] reasoned, did the continents on a longer time-scale -- making Teisserenc de Bort not only the discoverer of the stratosphere but an honorary grandfather of continental drift."
Nature http://www.nature.com/nature
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